This invention relates to a circuit arrangement for two-wire full-duplex data transmission via two lines from a station comprising a transmitter and receiver to another station also comprising a transmitter and receiver.
In a known data transmission system, transmitters and receivers are each arranged in first and second stations, and, using four-wire operation, the transmitter in the first station is connected by two lines to the receiver in the second station, and the transmitter in the second station is connected by two further lines to the receiver in the first station. In this system the internal resistance of the transmitter amounts to approximately 0 Ohms, and the internal resistance of the receiver amounts to approximately 200 Ohms. Under this condition a favorable pulse rise time characteristic is achieved for the transmission of pulse-like data signals, and this characteristic results in low telegraphic distortions.
In another known data transmission system, in each case a transmitter and a receiver are arranged both in the region of a first station and a second station, but in this case the two stations are connected to one another during two-wire operation by two lines. The transmitter and the receiver in each station are each connected to a bridge circuit. Two resistors each having a value of 200 Ohms form a first and a second bridge resistance. The third bridge resistance is formed by the two lines and by the circuit arrangements of the other station which are connected to the lines. The fourth bridge resistance is formed by a balancing resistor. The transmitter is connected at two diagonal points of this bridge circuit, and the receiver is connected to the two other diagonal points.
As is true in four-wire operation, an obvious course for two wire operation would be to dimension the first and second bridge resistor at 200 Ohms. However, since in two-wire operation the transmitter in a station is connected in series with the second bridge resistor of 200 Ohms, then, considered from the other station, the transmitter has an internal resistance of 200 Ohms. This results in a substantially impaired pulse rise time characteristic in comparison to the conditions of four-wire operation.
In order to achieve a similar pulse characteristic for two-wire operation, as in four-wire operation, an obvious course would be to dimension both the first and the second bridge resistors in the two stations at in each case 100 Ohms. However, this measure would have the disadvantage that the maximum range in two-wire operation would thereby be reduced to half the maximum range in four-wire operation.
An object of the invention is, therefore, to provide a circuit arrangement which, in simple fashion, permits the use of two-wire operation, instead of four-wire operation, without any reduction in the maximum range of transmission during two-wire operation and without increasing the telegraphy distortions.